Non Destructive Method Theory - Basic Principles - https://www.tinker.af.mil/Portals/106/Documents/Technical%20Orders/AFD-101516-33B-1-1.pdf AF338-1-1-EC-CP4Sc0-Indice ROCarneval

Capítulo 4 - MÉTODO DE INSPEÇÃO POR CORRENTES PARASITAS

traduzido do livro: AIR FORCE TO 33B-1-1 / ARMY TM 1-1500-335-23 / NAVY (NAVAIR) 01-1A-16-1 - Manual Técnico - Métodos de Inspeção Não Destrutiva, Teoria Básica

  1. APARELHO DE EC
    1. Componentes do Sistema de Inspeção por Correntes Parasitas
      1. Oscilador
      2. Bobina de Ensaio (Sonda)
      3. Circuito em Ponte
      4. Circuito de Processamento do Sinal
      5. Mostrador/Tela de Resposta
    2. Subsistemas do Ensaio de Correntes Parasitas
      1. Sondas (Montagem da Bobina)
        1. Isolamento da Sonda
        2. Tipos e Classificação das Sondas
          1. Modo de Operação
          2. Método de Aplicação da Sonda
          3. Considerações e Limitações do Projeto da Sonda
          4. Usos e Limitações das Sondas Internas e Externas
        3. Funções do Instrumento de Correntes Parasitas
          1. Requisitos Gerais
          2. Requisitos Específicos da Instda Instrumentação
          3. Componentes da Instrumentação
          4. Osciladores de Frequência Variável
          5. Circuito em Ponte
          6. Circuitos de Amplificação
          7. Forma de Apresentação de Sinais e Telas
            1. Medidores (analógicos)
            2. Mostradores Digitais
              1. Mostradores Lineares com Base no Tempo (varredura)
              2. Aparelho de Correntes Parasitas com Plano de Impedância
        4. Aparelho Digital
        5. Gravadores
        6. Dispositivos de Varredura Mecânica
        7. Posicionadores e Guias
        8. Processos Especiais
          1. Detecção por Amplitude
          2. Ensaio Multifrequência
          3. Técnica de Corrente Parasita Pulsada
          4. Medidas da Espessura do Metal
          5. Corrente Parasita de Baixa Frequência
          6. Ensaio com Dupla Frequência
        9. Técnicas Eletromagnéticas Fortemente Relacionadas com Corrente Parasita
          1. Ensaio de Ruído Barkhausen em Materiais Ferromagnéticos
          2. Imagem Ótico-magnética ("MOI")
        10. Aplicações de Técnicas Avançadas

4 ET EQUIPMENT.
Most eddy current nondestructive test instruments for field use are portable AC or battery powered units. They are generally lightweight, less than 6 lbs., with batteries that provide up to 12 hours of operation. They can have a type of digital display such as liquid crystal display (LCD), or electroluminescent (EL) display. Some units have dual frequency operations with in terchangeable display features. Newer units have state-of-the-art circuitry with advanced microprocessors. Frequency ranges of approximately 100 Hz to 6 MHz for detection of large and minute discontinuities. These units can be used to inspect f irst and second layer cracks, coating, plating thicknesses, and conductivity testing.


4.1 Components of an Eddy Current System.
In its simplest form, an eddy current inspection system consists of the following components:
An oscillator
A coil assembly
A bridge circuit Signal processing circuits
An output display (readout/screen

A block diagram of an inspection system is shown in Figure 4-25 with the coil applied to a test part. Systems may be con structed for multiple purposes or for very specialized functions. In general, instruments designed for specific tasks, such as measuring coating thickness or electrical conductivity, are easier to calibrate and operate than general-purpose instruments but also are limited to their designed application

4.1.1 Oscillator.
The oscillator provides an alternating current of one or more frequencies to the test coil. The frequency used is determined by the intent of the inspection and the material being inspected. Frequencies used for ET range from less than 100 Hz to greater than 6 MHz.

4.1.2 Coil Assembly (Probe).
The coil assembly induces eddy currents into the part being inspected and detects changes in eddy current flow. For some applications, a single coil is used for both functions. More commonly, multiple coils are employed in an assembly. A common configuration has one coil inducing the eddy current flow and separate coils used as detectors. Another configuration uses one coil as both an inducer and a detector on the test part

4.1.3 Bridge Circuit.
The bridge circuit converts changes in eddy current magnitude and distribution into signals that are ultimately processed and displayed. A common mode of operation is to have the output of the bridge equal zero for a good or non-flaw condition. Presence of a flaw or an other-than-good condition results in an unbalance of the bridge, thus producing a relatively small signal. This signal becomes the input to subsequent circuits

4.1.4 Signal Processing Circuits
The processing of the signal from the bridge circuit depends on the type of informa tion to be displayed. Simple eddy current devices can be built that detect and amplify the signal or convert the signal into digi tal format (e.g., a conductivity value). More sophisticated systems can process the complex electromagnetic signal into am plitude and phase, and provide filtering to suppress unwanted signals. Details of the processes are discussed further in later sections.

4.1.5 Output Display.
Eddy current test data can be presented in analog or digital format. Some common output devices are meter readout, a strip chart, an X-Y recorder plot, or digital display. Meters are suitable for performing specific types of tests requiring a measurement of signal amplitude only. Strip charts, X-Y recorders, and digital storage allow the signal ampli tude to be displayed and correlated with some other parameter such as time or position. Eddy current instruments with a two-dimensional graphical display are used where both the eddy current signal amplitude and phase must be measured. These are the most common instruments available, and provide the inspector with the greatest capability to interpret results.

Diagrama de blocos dos aparelhos de CP
Figure 1. Block Diagram of ET System


4.2 Eddy Current Subsystems.
Eddy current systems generally consist of three subsystems. One is the probe or probe subsystem. Second is the eddy current instrument. The third is the accessory subsystem. Scanners and recorders are in cluded with some subsystems and are considered to be accessories.

4.2.1 Probes (Coil Assemblies).
Eddy current probes consist of one or more coils designed to induce eddy currents into a part being inspected and detect changes within the eddy current field. A fundamental consideration in selecting an eddy current probe is its intended use. A small diameter probe or narrow encircling coil will provide increased resolution of small defects. A larger probe or wider encircling coil will provide better averaging of bulk properties with a loss in sensitivity to small defects. Also the probe or coil must match the impedance range of the eddy current instrument with which it is to be used.

4.2.1.1 Probe Shielding.
Probe shielding is used to prevent or reduce the interaction of the probe’s magnetic field with nonrelevent features in close proximity of the probe. Shielding could be used to reduce edge effects when testing near di mensional transitions such as a step or an edge. Shielding could also be used to reduce the effects of conductive or magnetic fasteners in the region of testing. Eddy current probes are most often shielded using magnetic shielding or eddy current shielding.

4.2.1.1.1 Magnetically shielded probes have their coil surrounded by a ring of ferrite or other material with high perme ability and low conductivity. The ferrite creates an area of low magnetic reluctance and the probe’s magnetic field is con centrated in this area rather than spreading beyond the shielding. This concentrates the magnetic field into a tighter area around the coil

4.2.1.1.2 Eddy current shielding uses a ring of highly conductive but nonmagnetic material, usually copper, to surround the coil. The portion of the coil’s magnetic field that cuts across the shielding will generate eddy currents in the shielding material rather than in the nonrelevent features outside of the shielded area. The higher the frequency of the current used to drive the probe, the more effective the shielding will be due to the skin effect in the shielding material.

4.2.1.2 Classification of Probes.
Eddy current probes and coils can be classified by mode of operation, application, or design.

4.2.1.2.1 Mode of Operation.
There are three general modes of operation for eddy current coil assemblies; absolute, differential, or driver/receiver (also called reflection)
  • The most common type of eddy current probe used in field applications is the absolute probe. Absolute probes consist of a single coil that is placed in contact with, or adjacent to, the part being inspected. Since any changes in the area interrogated by the coil produce a response, absolute probes can be used to measure specific materials properties such as electrical conductivity and magnetic permeability. They may have other discrete electrical elements such as ca pacitors included in the probe housing for matching to specific equipment requirements.
  • Differential probes contain two or more coils and are intentionally designed to produce a response when changes are sensed by the active coil only. Consequently, if the differential probe has two coils mounted side by side, gradual changes in electrical conductivity or magnetic permeability would be sensed by two coils simultaneously and no re sponse would occur. On the other hand, if an abrupt change in conductivity should occur, localized to where it can be sensed by only one coil at a time, then there would be a response. Normally, in both surface and bolt hole differen tial probes, two small sensing coils are wound side-by-side in the shape of two back-to-back capital D’s. They are wired in series, with one wound clockwise and the other counterclockwise. This produces an indication from a crack that deflects first one way, then the opposite way, while producing little or no indication from conditions that affect both coils equally, like lift-off or conductivity change.
  • Reflection probes can have a wide variety of configurations, but all have a driver coil wired separately from one or more receiver coils. A probe with one receiver coil is called reflection-absolute , and a probe with two receiver coils is reflection-differential . Reflection probes generally deliver better signal-to-noise levels, but are harder to make and therefore more expensive.
  • A fourth type of probe, remote field, has two or more coils, with the driver coil being a distance from the receiver coil(s). Remote field eddy current probes are used for deep penetration into thicker structures
4.2.1.2.2 Method of Probe Application. Eddy current probes can also be classified by the method of application Figure 4-26). The most common application is the contact or surface probe used for flat or relatively flat surfaces of a part. Eddy current probes used to encircle a part are called encircling coils. Eddy current probes completely encircled by the part are called ID coils or bobbin coils. Through-transmission probes, which utilize a coil on each side of a part (a sheet of alumi num for instance) is another method of application. All of these probe applications can be operated in absolute or differen tial modes (Figure 4-27). Eddy current probes can also be classed according to the shape or some other prominent feature of the probe. Very thin probes are called pencil probes. Probes with special electromagnetic shielding are called shielded or focused probes. Probes used in rivet or bolt holes are called bolt hole probes. Certain types of probes with shaped ferrite cores may be referred to as E-core, U-core, and pot or cup core probes.

4.2.1.2.3 Probe Design Considerations and Limitations. Eddy current probes have several conflicting requirements. First, they must be a reasonable match to the electrical impedance requirements of the instrument to which they are con nected. The closer the impedance match, the higher the signal-to-noise ratio. Also, the coils need to be designed for the flaw size to be detected. Smaller flaws require smaller coils. Most eddy current testing in the field is accomplished with surface probes. The surface probe is used on plates, sheets, irregularly shaped parts, and in holes. The extent of the area to be tested by the probe is controlled by the coil diameter and by the presence of coil shielding. When the area to be scanned is large, pancake-type surface coils or overlapping multi-coil probes can be used to reduce the time required to inspect the part. When small flaws must be detected, coils, as small as 1/32 inch in diameter, can be used to examine limited areas.

Tipos de sonda quanto a forma
Figure 2. Basic Coil Configuration

Esquema elétrico de sondas Absoluta e Diferencial
Figure 4-27. Example of Absolute and Differential Mode

4.2.1.2.4 Use and Limitations of ID and Encircling Coils. An inside diameter (ID) coil may be used on tubes, pipes, or other cylindrical parts where the geometry is regular and the interior is accessible. The ID coil should nearly fill the part opening in order to provide a high fill-factor for maximum test sensitivity. The use of ID coils can be restricted by bends or non-uniform diameters. Encircling coils are used primarily for inspecting rods, tubes, cylinders, or wire in manufacturing applications. With both encircling and inside coils, the entire circumference of the specimen is evaluated at one time. Con sequently, while the axial location of defects (along the part length) can be determined, circumferential location (around the part) cannot be defined.

4.3 Functions of the Eddy Current Instrument.
The eddy current test instrument performs three basic functions. First, it generates the alternating current that induces the eddy current flow in the part to be inspected. Second, it processes the re sponses to the induced eddy current flow. Third, it displays the responses in a manner to aid interpretation.
  • Current Generators. The current generator is usually a variable frequency oscillator operated at a single frequency for any given inspection. Most instruments have the capability of operating at frequencies from 100Hz to 6 MHz. Newer in struments have the ability to provide multiple frequencies to the test coil(s), either sequentially or simultaneously.
  • Processing. The processing function of the eddy current instrument includes a number of sub-functions. Most instru ments include some form of a balancing or compensating circuit which is adjusted to provide essentially a zero out put for non-flaw conditions. The signal from the bridge circuit is amplified before proceeding to the detector and/or analysis circuitry. Signals can be analyzed for their amplitude and phase. The output from the analysis circuits may be further filtered to assist interpretation before display. c.
  • Display Methods. The primary display method of most eddy current devices is either one dimensional, such as a meter, or two-dimensional, such as an LCD screen. The outputs can also be transferred to X-Y recorders, strip chart record ers, magnetic storage media or even computers to both generate inspection records as well as aid in the analysis of the eddy current signals
4.3.1 General Requirements.
Eddy current instrumentation is the core of an eddy current system, whether the system is a simple instrument/coil combination or a fully automated scanning inspection station. To assure reliable operation, the in strumentation must have the capabilities described below:

  • Sensitivity. A term that refers to the instruments capability to find the most difficult to locate flaws; with reference to the size and type that need to be detected.
  • Low Noise. The noise should be low enough so the signal from the smallest flaw to be found (or smallest calibration f law) is at least three times the noise level of the instrumentation.
  • Response Time. The response time of the circuitry must be fast enough to process and display signals at the required scanning rate.
  • Selectivity. The instrumentation should be immune to external sources of electromagnetic interference.
  • Stability. The instrumentation display should remain frequency drift-free, during the required testing period.
  • Ruggedness. The instrumentation must be capable of operating in the test environment. This may include a variety of environmental extremes of temperature, humidity, dust, and vibration.

4.3.2 Specific Instrumentation Requirements.
Choice of an eddy current test instrument must take into account the type of flaw to be detected, the permeability of the material (nonferromagnetic or ferromagnetic), type of probe to be used, display method (meter, digital display, recorders, etc.), test frequency, and signal processing requirements, portability, if needed, and any accessories to be used.

4.3.3 Instrumentation Components.
In general, most eddy current instruments consist of an oscillator, a bridge circuit or similar null balancing system, and a variety of other circuits for processing and display of the eddy current signal. Units will vary depending upon the complexity of the instrumentation and the requirements of the test.

4.3.4 Variable Frequency Oscillator.
A basic eddy current instrument, while operating at a single frequency during a particular test, usually has an operating frequency range that is adjustable to meet a large variation of inspection situa tions. Low frequencies increase depth of penetration and consequently would be used for subsurface flaw detection in high conductivity materials. Higher frequencies limit depth of penetration and thus are used for low conductivity materials as well as for detecting smaller flaws. Some instruments also incorporate a fine adjustment of frequency as a mechanism for suppress ing lift-off. These instruments incorporate the probe coil in parallel with a capacitor as one leg of a bridge. The coil/capacitor combination is resonant near the intended operating frequency. The frequency selected for operation causes a meter deflec tion off-resonant enough to where lift-off causes less of an impedance change than caused by a defect and the impedance change for increasing lift-off is opposite to that for a defect.

4.3.5 Bridge Circuit.
A basic bridge circuit is shown in Figure 4-28. In this example, a voltage is applied at points E1 and E2 to the bridge containing impedances Z1, Z2, Z3, and Z4. Z1 and Z4 are fixed impedances of the same value; Z3 is an adjustable impedance; and Z2 the unknown or test probe impedance. Initially, Z3 is adjusted so that no current flows through the amplifier. This means the voltage at points A and B is the same and the bridge is said to be balanced or nulled. Any change in impedance of Z2, the test probe impedance, will result in a current change through the leg of the bridge and consequently a change in the voltage at point B. A current will then flow through the amplifier, since a voltage or potential difference exists between points A and B. The bridge is now said to be unbalanced. The bridge can again be balanced by ad justment of Z3 and the change in the test probe impedance, Z2, may be determined by measuring the change in Z3 required to rebalance the bridge. The bridge circuit in an eddy current test instrument is termed an impedance bridge since the circuit contains both resistive and reactive elements. Impedance Z2 in Figure 4 would consist of the eddy current test coil. Other reactive elements, inductors, and capacitors may be included in the impedance bridge depending upon the specific design and function. However, the basic principle is that a change in impedance of the test coil results in an imbalance of the bridge circuit. The output (imbalance) from the bridge circuit can be amplified, processed and displayed.

4.3.6 Amplification Circuits.
The imbalance in the bridge circuit is due to an impedance change at the test probe. It re sults in a change in signal amplitude, signal phase or both. These signal changes must be amplified, detected or demodu lated, and processed for presentation on the output device (meter, scope, or recorder, etc.). The flaw signal may be only several micro volts in amplitude and may require amplification of one thousand to one million times for further processing and dis play. The frequency content of the flaw signal can range from very low (essentially DC) to the maximum operating frequency of the eddy current instrument. This defines the distortion-free frequency response of the amplifier. The amplifier must also be very stable with very little drift in order to maintain the required sensitivity and calibration throughout the duration of the test

Circuito em Ponte de Wheatstone
Figure 4-28. Basic Bridge Circuit
















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